1. Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to the transmission of sounding reference signals in an uplink of a communication system.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys transmission signals from a Base Station (BS), or NodeB, to User Equipments (UEs). The communication system also includes an UpLink (UL) that conveys transmission signals from UEs to the NodeB. A UE, which is also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be embodied as a wireless device, a cellular phone, or a personal computer device. A NodeB is generally a fixed station and may also be referred to as an access point or other equivalent terminology.
A UL conveys transmissions of data signals carrying information content, of control signals providing control information associated with transmissions of data signals in a DL, and of Reference Signals (RSs), which are commonly referred to as pilot signals. A DL also conveys transmissions of data signals, control signals, and RSs. UL signals may be transmitted over clusters of contiguous REs using a Discrete Fourier Transform (DFT) Spread Orthogonal Frequency Division Multiplexing (DFT-S-OFDM) method. DL signals may be transmitted using an OFDM method.
UL data signals are conveyed through a Physical Uplink Shared CHannel (PUSCH) and DL data signals are conveyed through a Physical Downlink Shared CHannel (PDSCH).
In the absence of a PUSCH transmission, a UE conveys UL Control Information (UCI) through a Physical Uplink Control CHannel (PUCCH). However, when there is a PUSCH transmission, a UE may convey UCI together with data through the PUSCH.
DL control signals may be broadcast or sent in a UE-specific nature. Accordingly, UE-specific control channels can be used, among other purposes, to provide UEs with Scheduling Assignments (SAs) for PDSCH reception (DL SAs) or PUSCH transmission (UL SAs). The SAs are transmitted from the NodeB to respective UEs using DL Control Information (DCI) formats through respective Physical DL Control CHannels (PDCCHs).
A UE transmits an RS to either enable a NodeB to perform coherent demodulation of transmitted data or control signals, or to obtain measurements for a UL channel medium the UE experiences. An RS used for De-Modulation is referred to as a DM RS while an RS used for measurements of a UL channel medium is referred to as a Sounding RS (SRS).
FIG. 1 is a diagram illustrating a PUSCH structure over a Transmission Time Interval (TTI).
Referring to FIG. 1, a TTI consists of one subframe 110 that includes two slots 120. Each slot 120 includes NsymbUL symbols 130 for transmission of data information signals, UCI signals, or an RS. Some symbols in each slot are used to transmit a DM RS 140 that enables channel estimation and coherent demodulation of received data and/or UCI signals. The transmission BandWidth (BW) consists of frequency resource units, which are referred to as Resource Blocks (RBs). Each RB consists of NSCRB sub-carriers, or Resource Elements (REs) and a UE is allocated MPUSCH RBs 150 for a total of MSCPUSCH=MPUSCH·NSCRB REs for the PUSCH transmission BW.
The last subframe symbol may be used for transmission of an SRS 160 from one or more UEs. The SRS transmission parameters for a UE are configured by a NodeB through higher layer signaling, such as, for example, Radio Resource Control (RRC) signaling. The number of subframe symbols available for data transmission is NsymbPUSCH=2·(NsymbUL−1)−NSRS, where NSRS=1 if the last subframe symbol is used for SRS transmission, and otherwise NSRS=0.
FIG. 2 is a diagram illustrating a configuration for SRS transmissions from multiple UEs.
Referring to FIG. 2, an SRS transmission 260, 265 occurs in the last subframe symbol every 2 subframes 201, 203. PUSCH transmission of UE1 data 210 and UE2 data 220 are multiplexed in different BW parts during the first subframe 201, while the UE2 data 220 and UE3 data 230 are multiplexed during a second subframe 202, and UE4 data 240 and UE5 data 250 are multiplexed during a third subframe 203. In some symbols, UE1, UE2, UE3, UE4, and UE5 transmit DM RSs 215, 225, 235, 245, and 255, respectively. UEs with SRS transmissions may or may not have PUSCH transmissions in a same subframe and, if they co-exist in the same subframe, SRS and PUSCH transmission may be located at different parts of an operating BW.
A UE may transmit DMRS or SRS through the transmission of a respective Constant Amplitude Zero Auto-Correlation (CAZAC) sequence such as a Zadoff-Chu sequence. For a UL system BW consisting of NRBmax,UL RBs, a sequence ru,v(α) (n) can be defined by a Cyclic Shift (CS) α of a base sequence ru,v(n) according to ru,v(α) (n)=ejan ru,v(n), 9≦n<MSCRS, where MSCRS=mNSCRB is the length of the sequence, 1≦m≦NRBmax,UL, and ru,v(n)=xq(n mod NZCRS) where the qth root Zadoff-Chu sequence is defined by
                    x        q            ⁡              (        m        )              =          exp      ⁡              (                                            -              j                        ⁢                                                  ⁢            π            ⁢                                                  ⁢                          qm              ⁡                              (                                  m                  +                  1                                )                                                          N            ZC            RS                          )              ,0≦m≦NZCRS−1 with q given by q=└ q+½┘+ν·(−1)└2 q┘ and q given by q=NZCRS·(u+1)/31. The length NZCRS of the Zadoff-Chu sequence is given by the largest prime number such that NZCRS<MSCRS. Multiple RS sequences can be defined from a single base sequence through different values of α.
FIG. 3 is a diagram illustrating an SRS transmitter structure at a UE.
Referring to FIG. 3, by choosing non-consecutive REs to a frequency domain version of a CAZAC sequence, a comb spectrum can be obtained which is useful for orthogonally multiplexing, in the same symbol, (through frequency division) SRS transmissions with unequal BWs. Such SRSs are constructed by CAZAC sequences of different lengths, which cannot be orthogonally separated using different CSs. An SRS transmitter 310 generates a frequency domain CAZAC sequence. A selector 320 selects REs in an assigned transmission BW (including non-consecutive REs in case of a comb spectrum) at a sub-carrier mapper 330. An Inverse Fast Fourier Transform (IFFT) unit 340 performs an IFFT. A CS 350 unit applies the CS. The resulting signal passes though a Cyclic Prefix (CP) unit 360 which inserts a CP, and a time windowing filter 370. The resulting signal 380 is transmitted. For brevity, though additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas may be included, they are not shown.
FIG. 4 is a diagram illustrating an SRS receiver structure at a NodeB.
Referring to FIG. 4, after an antenna receives a Radio-Frequency (RF) analog signal and after passing further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters), a resulting signal 410 passes through a time-windowing filter 420 and a CP removal unit 430. Subsequently, a CS unit 440 removes a CS applied to a transmitted CAZAC sequence, a Fast Fourier Transform (FFT) unit 450 applies an FFT, a selector 460 selects received REs at a sub-carrier demapper 465, and a correlator 470 performs a correlation with a CAZAC sequence replica 480. Finally, output 490 can be passed to an UL channel estimator.
Several combinations for an SRS transmission BW can be supported as shown in Table 1. A NodeB may signal a configuration c through a broadcast channel, for example 3 bits can indicate one of eight configurations. The NodeB can then individually assign to each UE, for example using higher layer signaling of 2 bits, one of the possible SRS transmission BWs mSRS,bc (in RBs) by indicating a value of b for configuration c. Therefore, the NodeB may assign any of the SRS transmission BWs mSRS,0c, mSRS,1c, mSRS,2c, and mSRS,3c (b=0, b=1, b=2, and b=3, respectively, in Table 1) to a UE.
TABLE 1Example of mSRS,bc RB values for UL BW ofNRBUL RBs with 80 < NRBUL ≦ 110.SRS BW configurationb = 0b = 1b = 2b = 3c = 09648244c = 19632164c = 28040204c = 37224124c = 46432164c = 56020Not Applicable4c = 64824124c = 7481684
FIG. 5 is a diagram illustrating a structure for multiple SRS transmission BWs with configuration c=3 from Table 1.
Referring to FIG. 5, PUCCH transmissions 502 and 504 are located at the two edges of an operating BW and a UE is configured SRS transmission BWs with mSRS,03=72 RBs 512, mSRS,13=24 RBs 514, mSRS,23=12 RBs 516, or mSRS,33=4 RBs 518. A few RBs, 506 and 508, may not be sounded, but a NodeB may still be able to schedule PUSCH transmissions in those RBs as a respective UL SINR may be interpolated from the nearest RBs with SRS transmission. For SRS BWs other than the maximum one, a NodeB is assumed to assign to a UE a starting BW position for an SRS transmission.
SRS transmissions can be periodic or aperiodic (dynamic). For periodic SRS transmissions, a NodeB configures the SRS transmission parameters for each UE through higher layer signaling, such as, for example, RRC signaling. Periodic SRS transmissions remain valid until re-configured, again through higher layer signaling. The periodic SRS transmission parameters may include an SRS BW, a comb, a CS, a starting BW position, a period (for example, one SRS transmission every 5 subframes), a starting subframe (for example, the first subframe in a set of subframes supporting SRS transmission), a number of UE transmitter antenna ports transmitting SRS, and a part of an operating BW an SRS transmission may hop. As periodic SRS transmissions are semi-statically configured, they may consume considerable UL resources, especially if they have a large transmission BW and need to be supported from multiple UE transmitter antenna ports. To dynamically control the UL resources allocated to SRS transmissions, aperiodic (dynamic) SRS transmissions can be utilized.
Aperiodic SRS transmissions are triggered by physical layer signaling (PDCCH) through some of the DCI formats that schedule PUSCH transmissions from a UE or PDSCH receptions to a UE. These DCI formats include an aperiodic SRS request field, which instructs a UE to transmit aperiodic SRS at a predetermined subsequent UL subframe relative to a DL subframe of the DCI format reception. For example, this UL subframe can be a first UL subframe supporting aperiodic SRS transmissions that occurs at least four TTIs after an UL subframe associated with a DL subframe of the DCI format transmission.
One or more sets of aperiodic SRS transmission parameters can be configured to a UE for each DCI format. An aperiodic SRS request field in a respective DCI format can indicate which set a UE should use for an aperiodic SRS transmission. The aperiodic SRS transmission parameters may be the same or a sub-set of the periodic SRS ones. For example, an aperiodic SRS transmission subframe may be uniquely determined, relative to a subframe of a respective DCI format reception, and an aperiodic SRS transmission may occur only once (no need to define a transmission period).
Table 2 shows an interpretation of an aperiodic SRS request field when it consists of 1 bit or when it consists of 2 bits. For one value of an aperiodic SRS request field, there is no aperiodic SRS transmission while the other values indicate aperiodic SRS transmission and a corresponding configuration of the aperiodic SRS transmission parameters.
TABLE 2Aperiodic SRS request field operation.Aperiodic SRS Request Field -1 BitAperiodic SRS Request Field - 2 Bits0: No aperiodic SRS00: No aperiodic SRS transmissiontransmission1: Aperiodic SRS Transmission01: Aperiodic SRS Transmission usingusing configuration 0configuration 1—10: Aperiodic SRS Transmission usingconfiguration 2—11: Aperiodic SRS Transmission usingconfiguration 3
Multiple DCI formats, each scheduling PDSCH or PUSCH for each of the potentially multiple transmission modes supported by a UE, may include an aperiodic SRS request field. Moreover, different DCI formats may be associated with different configurations of aperiodic SRS transmission parameters. The size of an aperiodic SRS request field (for example, 1 bit or 2 bits) can be different for different DCI formats. As a UE may receive multiple such DCI formats for which an aperiodic SRS transmission instance (PUSCH symbol in an UL subframe) is the same, a UE behavior in responding to respective multiple aperiodic SRS request fields needs to be determined so that proper operation can be achieved. Moreover, if a NodeB expects a UE to transmit an aperiodic SRS and a UE does not, unless a NodeB can perform reliable detection for the absence of an aperiodic SRS transmission, an estimate of a UL channel medium will be incorrect. This can adversely affect several subsequent scheduling decisions a NodeB may make for PUSCH transmission to the detriment of UL system throughput and communication quality with the particular UE.